1. Field of the Invention
The invention relates to an optical disc and a manufacturing method thereof, particularly to an optical disc with a printed vibration suppressing layer, and a manufacturing method thereof.
2. Description of Related Art
It is widely known that an optical disc, while being portable, can store a large amount of data in various different formats (e.g., video clips, music, documents, graphics), and it has become an important information storing means nowadays. The optical disc industry has been pursuing to manufacture low-cost, high read/write quality optical discs.
A conventional optical disc has a thickness of about 1.2 mm. As the manufacturing techniques advance, a material-saving optical disc having a thickness of only 0.6 mm has been disclosed in the literature. Generally, an optical disc rotates at a speed roughly between 2,400 and 3,600 rpm for reading or writing data. However, once the rotation speed exceeds 3,000 rpm for a faster access of data, vibration of the disc can occur. The vibration causes instability of the rotating disc, thus causing read/write problems. While the optical disc is made thinner, resulting in a lighter disc weight, the vibration will affect the optical disc even more. To reduce the vibration, a damping layer can be provided on the optical disc to reduce the duration affected by the vibration during operation. However, the related art does not disclose the best distribution region of the damping layer on the disc.
U.S. Pat. No. 5,458,940 and U.S. Pat. No. 6,461,711 provide solutions to the vibration problem mainly by using a damping layer provided over the recording layer of the optical disc to reduce the time affected by the vibration and to reduce the disc vibration amplitude. However, the damping layer can deform the disc, and the two aforementioned patents do not address this problem.
FIG. 1 is a schematic view of an optical disc having a damping layer. The optical disc 1 includes a substrate 13 and a damping layer 14. The substrate 13 includes an inner supporting section 11 and an outer information recording section 12, the former being thicker than the latter. The damping layer 14 is provided over the outer information recording section 12. The damping layer 14 may reduce the time the optical disc 1 is affected by the vibration and reduce the vibration amplitude of the disc. Namely, abating the instability of the disc to reduce the potential problems associated with the read/write operation. On the other hand, since the damping layer 14 and the substrate 13 are made of different materials, the bonding of the damping layer 14 to the substrate 13 results in a greater initial vertical deviation (abbreviated by “V.D.” hereinafter) when there is a difference between the room temperature and the processing temperature. The V.D. has a positive value if the optical disc 1 warps upward from a reference plane; otherwise, the V.D. has a negative value if the optical disc 1 warps downward from the reference plane. A typical range of V.D. for allowing a proper read/write operation is between ±300 μm. If the V.D. of the optical disc 1 exceeds the allowable range, there could be a read/write problem. Besides, the deformation of the optical disc can increase with time under varying humidity and temperature (environmental testing), leaving the originally readable/writable disc inoperative. This means the optical disc will lose its read/write function or the read/write quality will deteriorate.
Moreover, the bonding process of the damping layer cannot be satisfactorily adjusted to optimize the characteristics of the damping layer. Consequently, although attaching a damping layer on an optical disc can reduce the disc vibration, it causes undesirable deformation of the disc. Furthermore, if spin coating process is used to form a damping layer, a long processing time and more materials are required.
In addition, although a damping layer can reduce the time a disc is affected by vibration, it has a limited improvement on the degree of the deformation because the damping layer is made of a soft material. Accordingly, Taiwan Patent Application No. 96130848 discloses an optical disc bonded with a deformation-rectifying layer on the opposite side of the recording layer so as to reduce the disc deformation. However, the deformation-rectifying layer needs additional processing time, which is disadvantageous to mass production of discs.